Low bit rate digital speech is required where there is limited storage capacity for the speech signals, or where the transmission channels for carrying the speech signals have limited capacity such as high frequency communications, digital telephone answering machines, electronic voice mail, digital voice loggers, etc.
Two techniques that have been successful in producing reasonable quality speech at rates of approximately 4800 bits per second are referred to as Codebook Excited Linear Predictions (CELP) and Harmonic Coding, the latter defining a class which includes Multiband Excitation (MBE) and Sinusoidal Transformation Coders (STC).
A multiband excitation vocoder is described in an article by Daniel W. Griffin in IEEE Transactions on Acoustics, Speech and Signal Processing, vol. 36, no. 8, pp. 1223-1235, August, 1988.
CELP coders produce good quality speech at about 8 kbps. However as the bit rate decreases, the quality degrades gracefully. Below 4 kbps, the quality degrades more rapidly.
At low bit rates, Pitch-Excited LPC (PELP) coders operating at 2.4 kbps are currently the most widely used. However they suffer from major drawbacks such as unnatural speech quality, poor speaker recognition and sensitivity to acoustic background noise. Because of the nature of the algorithm used, the quality cannot be significantly improved.